Carbon nanotubes (CNTs), with their unique shapes and characteristics, are being considered for various applications. A carbon nanotube has a tubular shape of one-dimensional nature which is obtained by rolling one or more graphene sheets composed of six-membered rings of carbon atoms into a tube. A carbon nanotube which is formed from one graphene sheet is called a single-wall nanotube (SWNT) while a carbon nanotube which is formed from plural graphene sheets is called a multi-wall nanotube (MWNT). SWNTs are about 1 nm in diameter, while multi-wall carbon nanotubes measure several tens nm in diameter, and both are far thinner than their predecessors, which are called carbon fibers.
One of the characteristics of carbon nanotubes resides in that the aspect ratio of length to diameter is very large since the length of carbon nanotubes is on the order of micrometers. Carbon nanotubes are unique in their extremely rare nature of being both metallic and semiconductive because six-membered rings of carbon atoms in carbon nanotubes are arranged into a spiral. In addition, the electric conductivity of carbon nanotubes is very high and allows a current flow at a current density of 100 MA/cm2 or more.
Carbon nanotubes excel not only in electrical characteristics but also in mechanical characteristics. That is, the carbon nanotubes are distinctively tough, as attested by their Young's moduli exceeding 1 TPa, which belies their extreme lightness resulting from being formed solely of carbon atoms. In addition, the carbon nanotubes have high elasticity and resiliency resulting from their cage structure. Having such various and excellent characteristics, carbon nanotubes are very appealing as industrial materials.
Applied researches that exploit the excellent characteristics of carbon nanotubes have been heretofore made extensively. To give a few examples, a carbon nanotube is added as a resin reinforcer or as a conductive composite material while another research utilizes a carbon nanotube as a probe of a scanning probe microscope. Carbon nanotubes have also been utilized as minute electron sources, field emission electronic devices, and flat displays. An application that is being developed is to use a carbon nanotube as a hydrogen storage.
As described above, a carbon nanotube is expected to find use in various applications. In particular, applications thereof as electronic materials and electronic devices have been attracting attention. Electronic devices such as a diode and a transistor have been already prototyped, and are expected to replace current silicon semiconductors. In order to integrate such electronic devices using carbon nanotubes as those described above and to put the resultant into practical use, a wiring having a size of the order of 10−9 m (nm) is required. In general, metal is used as a current wiring material. However, there are physical limitations on the realization of a wiring having a size of the order of 10−9 m with a metal wiring.
In addition, the metal wiring involves a problem called migration. That is, metal wirings (gold, copper, aluminum, and the like) used in the current semiconductor devices each have a polycrystalline structure. In the structure, the orientation varies from crystal grain to crystal grain, so that a large number of defects are present at a grain boundary. Some of the metal atoms trapped in the defects have weak interatomic bonds. Owing to such structural factors, when a current density or a thermal stress acts while exceeding a certain tolerance, weakly bonded metal atoms start to move, and portions devoid of metal atoms (voids) form and grow. As a result, problems such as an increase in wiring resistance and disconnection occur (this phenomenon is referred to as “migration”)
On the other hand, as described above, a carbon nanotube is structured by rolling a graphene sheet, and hence has no grain boundary. Therefore, the carbon nanotube is not considered to involve the occurrence of migration. In actuality, Kazuhito Tsukakoshi and Katsunobu Aoyagi, Applied Physics Vol. 72, p. 333 (2003) describes that a current of 0.4 mA can be caused to stably flow through a carbon nanotube having a diameter of 10 nm. The current value is 2 or more orders of magnitude larger than a migration tolerance for a copper wire. Therefore, it can be said that a carbon nanotube is an extremely promising material as a wiring material.
However, it is extremely difficult to actually wire carbon nanotubes. At present, several techniques of wiring carbon nanotubes have been attempted.
A first technique includes: picking up one or several carbon nanotubes by using a manipulator in a scanning electron microscope; and arranging the one or several carbon nanotubes at a desired position. A technique for arranging carbon nanotubes by using a probe microscope may be given as an example of a modification of the first technique. However, the technique requires much time and labor. Therefore, the technique is suitable for fundamental studies but is not practical.
A second technique is a technique for orienting a carbon nanotube in a certain direction by using electrophoresis. With this technique, carbon nanotubes may be wired in one direction, but it is difficult to wire carbon nanotubes in plural directions. Thus, this technique is not realistic.
A third technique is a technique employing a chemical vapor deposition (CVD) method. The CVD method includes: using an acetylene gas or methane gas containing carbon as a raw material; and producing a carbon nanotube by a chemical decomposition reaction of the raw material gas. H. Dai, J. Kong, C. Zhou, N. Franklin, T. Tombler, A. Cassell, S. Fan, M. Chapline, J. Phys. Chem. 103, p. 11246-11255 (1999) discloses a technique of generating a carbon nanotube by: forming a pattern of a catalyst metal necessary for generating a carbon nanotube on a porous silicon substrate according to a semiconductor process; and causing a raw material gas to flow through the pattern. However, in the method, a carbon nanotube grows only in the direction perpendicular to the substrate.
Cassell, N. Franklin, T. Tombler, E. Chan, J. Han, H. Dai, J. Am. Chem. Soc. 121, 7975-7976 (1999) discloses a method of wiring a carbon nanotube horizontally to a substrate. That is, disclosed is a technique including: fabricating an Si pillar on a substrate; mounting a catalyst on the top part of the pillar; and allowing a methane gas to flow to bridge a carbon nanotube between pillars. The method by this technique has certainly enabled horizontal wiring. However, the probability of cross-link is extremely low, and wiring at an arbitrary position is still difficult.
As described above, a technique for wiring one or several carbon nanotubes is still at a developmental stage.
In the meantime, a method for wiring or patterning using a carbon nanotube as a film has been developed. For example, pattern formation of a carbon nanotube has been heretofore performed by using a screen printing method or a photolithography technique. Each of those techniques is excellent in forming a pattern in a wide area at once, and is used for patterning of an electron source in a field emission type display (FED). However, in each of those methods, a carbon nanotube is merely dispersed in a solvent before application, or is mixed with a binder before application. Therefore, the carbon nanotube is insufficient in terms of performance such as a mechanical strength or electric conductivity, and is hardly used directly as an electrode or an electric circuit.
Patent Document 1 discloses that a carbon nanotube with a three-dimensional structure can be formed by using a functionalized carbon nanotube. However, this publication discloses that, for simple use in a chromatography-flow cell electrode, a product obtained by depositing onto a metal mesh a carbon nanotube to which a functional group that is porous and serves to separate and suck a passing substance has been bonded is made porous, or carbon nanotubes are bonded to each other by using an alkoxide of aluminum or silica (the alkoxide itself serves as an insulator) as a cross-linking agent.
In the former example in JP 2002-503204 A, a wiring or the like to be obtained is merely a deposit of carbon nanotubes, and its shape can only be an inner shape of a vessel. In addition, when one tries to process such a carbon nanotube deposit, carbon nanotubes scatter during the processing, so that the deposit cannot be processed into a desired shape.
On the other hand, in the latter example in JP 2002-503204 A, in the case where cross-linking is performed using an alkoxide, alkoxides together form a complicated cross-linking structure, so that it is difficult to densely arrange carbon nanotubes. Therefore, carbon nanotubes are isolated and dispersed into multiply entangled residues of alkoxides even though the carbon nanotubes are chemically bonded. For example, when aluminum tributoxide is used as a cross-linking agent, even if carbon nanotubes are bonded through one aluminum structural unit (—O—Al—O—), several of or several tens of aluminum structural units are bonded in a chain fashion at other portions. As a result, a distance between carbon nanotubes at a cross-linking portion of carbon nanotubes varies. Therefore, although characteristics are uniformized in a structure having an entirely large size, the lower the size of a structure, the larger the variations in characteristics. As a result, there is a possibility that problems such as a problem in that excellent characteristics of a carbon nanotube itself are obtained only accidentally occur. As described above, a carbon nanotube structure in which carbon nanotubes are cross-linked by using an alkoxide and bonded in a network fashion is not suitable for thinning and downsizing while characteristics that are unique of a carbon nanotube are exerted. In addition, not only electrical applications of the structure but also general applications thereof are restricted.
Therefore, an object of the present invention is to solve the above problems involved in the prior art. More specifically, an object of the present invention is to provide a carbon nanotube device capable of efficiently exerting various electrical or physical characteristics of a carbon nanotube, a method of manufacturing the same, and a carbon nanotube transfer body facilitating the manufacture of a carbon nanotube device. In particular, an object of the present invention is to provide a carbon nanotube device capable of arranging a metal wiring or electrical component which is fine and has an arbitrary shape on the surface of a base body such as a substrate, a method of manufacturing the same, and a carbon nanotube transfer body.